Incased variable-voltage ratio transformer



April 14, 1-931.

INCASED E. D. TREANOR VARIABLE VOLTAGE RATIO TRANSFORMER Filed July 8. 1930 Invntow:

Edward 7 D. Treahor",

0% His Attorney.

Patented Apr. 14, 1931 UNITED STATES PATENT OFFICE T EDWARD D. TREANOK OF PITTSFIELD, MASSACHUSETTS, .ASSIGNOB. IO GENERAL ELEC- TRIO COMPANY, A CORPORATION OF NEW YORK INGAS ED VARIABLE-VOLTAGE RATIO TRANSFORMER Application filed July 8,

My invention relates to encased variable voltage ratio transformers. It is often desirable to provide a transformer with some means for changing the ratio between its r1- 6 mary and secondary voltages. This is en done by providing a transformer winding.

with taps which are connected to a tap switch so that the effective number of turns in the winding may be varied, thus changing the turn ratio and therefore the voltage ratio. In order that the leads between the tap switch and the transformer winding may be as short as possible, the tap switch 1s often enclosed with the transformer in a common grounded 7.5 metal casing which is usually filled or partially filled with an insulating liquid such as a suitable mineral oil having high dielectric strength. The space available for the switch is often very limited so that it is desirable that the switch be as compact as possible. It is often necessary also to locate the switch close to the inner surface of the casing.

The switch has contacts which are connected to differentpoints of the transformer winding. These contacts are thus maintained at different potentials and must be separated from each other sufficiently to prevent arcing and breakdown of the insulation between them. The voltages impressed on the 80 switch contacts produce an electric field surrounding t-hem. The adjacent grounded portion of the casing tends to distort this electric field so that a eater spacing of the contacts or at least o some of them and therefore a larger switch is necessary to prevent arcing between the contacts. The general object of the invention is to provide means for reducing or reventing this distortion of the electric field surrounding the switch contacts and thus to permit a switch of minimum size to be used.

The invention will be further explained by ,the following description taken 1n connection withthe accompanying drawing in which Fig. 1 is a view, partly broken away, of an encased variable ratio transformer provided with one form of the invention and Fig. 2 is an explanatory view.

Likereference characters indicate similar parts in both figures of the drawing.

1930. Serial No. 488,449.

The transformer 10 shown in Fig. 1 is immersed in a body of insulating liquid 11 in a metal casin 12. The transformer has a windin 13 w ich is provided with taps connecte by tap leads 14 to a plurality of fixed contacts 15 of a tap switch 16 by means of which the tap connections may be changed to vary the voltage ratio of the transformer. These changes of the tap connections are made by a movable contact 17 which may be operated by a shaft 18 extending through the cover of the transformer casing. The tap switch 16 is located close to the tapped portion of the transformer winding 13 so that the leads 14 may be as short as possible, the switch therefore being at one side of the transformer and close to the adjacent wall of the casing 12.

The fixed contacts 15 are maintained at the voltages of the points in the transformer 7 winding 13 to which they are connected and these contacts are surrounded by an electric field due to the voltages impressed on them. The adjacent wall of the casing 12 is composed of conductive material which is at ground potential and which is at different distances from the various contacts 15. This tends to distort the electric filed surrounding the contacts 15 and any distortion of this electric field increases the necessary spacing between the contacts or at least between some of them and therefore makes the use of a larger switch necessary.

Distortion of the electric field surrounding the switch contacts 15 is prevented or largely reduced by a conductive shield 19 lnterposed between these contacts and the adjacent wall of the casing 12. This shield 19 is preferably in the form of a thin metal sheet embedded in solid insulation and connected by a conductor 20 with one of the contacts 15. The shield 19 is therefore maintamed at a voltage which is the same as that of one of the contacts 15 and which does not differ nearly as much from the voltage of any of these contacts as does the voltage of the grounded casing 12. The shield 19 thus greatly reduces the distortion of the electric eld surrounding the contacts and makes possible a closer spacing of the contacts and a smaller switch. This often makes ossible the use of a smaller casing 12 and t us a smaller quantity of insulating liquid 11 within the casing.

The invention has been explained by describing and illustrating a particular form and application thereof but it will be apparent that various changes may be made without departing from the spirit of the invention or the scope of the appended claims.

What I claim as new and desire to secure by Letters Patent of the United States, is

l. A transformer having a winding with taps, a switch having contacts connected to said taps, a conductive casing enclosing said transformer and switch, a conductive shield between said switch and an adjacent portion of the inner surface of said casing to reduce distortion of the electric field surrounding said contacts, and means for maintaining the voltage of said shield above that of said casing.

2. A transformer having a winding with taps, a switch having contacts connected to said taps, a conductive casing enclosing said transformer and switch, a conductive shield between said switch and an adjacent portion of the inner surface of said casing to reduce distortion of the electric field surrounding said contacts, and means for maintaining the voltage of said shield near the voltages of said contacts.

3. A transformer having a winding with taps, a switch having contacts connected to said taps, a conductive casing enclosing said transformer and switch, and a conductive shield between said switch and an adjacent portion of the inner surface of said casing to reduce distortion of the electric field surrounding said contacts, said shield being conductively connected to one of said contacts to maintain the voltage of the shield above that of said casing.

In witness whereof, I have hereunto set my hand this 2nd day of July, 1930.

EDWARD D. TREANOR. 

